This invention relates to an improved product recovery tube for use in recovering polysilicon from a fluidized bed reactor. High purity monocrystalline silicon is in great demand as a semiconductor material. The purity of the silicon is critical as impurities, especially metal impurities, in amounts as small as a few parts per billion, can adversely alter the silicon's conductivity or electronic characteristics.
Most of the world's supply of semiconductor grade monocrystalline silicon is produced from polycrystalline silicon, i.e., polysilicon, which in turn is produced from the thermal decomposition of a silicon source, e.g., silane, monochlorosilane, and the like. The thermal decomposition can be efficiently carried out in a fluidized bed reactor which accepts, as a feed, a mix of a silicon source gas and a carrier gas. The silicon source gas decomposes to yield silicon which deposits on silicon seed particles which are maintained in a fluidized state. As deposition occurs, the seed particles grow until they are of the desired size. Removal of the grown particles from the fluidized bed reactor is accomplished by providing a product withdrawal tube which extends downwardly from the gas distributor in the reactor to a separate cooling vessel which holds a charge of previously withdrawn polysilicon. Gas is fed upwardly in the product withdrawal tube so as to fluidize at least a portion of the polysilicon which is present in the tube. The fluidization in the tube serves at least two important functions: (1) fluidization tends to minimize the tendency of the polysilicon particles to fuse one to the other in the presence of decomposing silicon source gas, and (2) since fluidization facilitates heat transfer, there is a smaller temperature difference between the polysilicon in the tube nearest the reactor and the polysilicon in the tube farthest from the reactor and, thus, there is a diminishment of thermal shock when the polysilicon is discharged from the tube.
To effect removal of the grown polysilicon from the fluidized bed reactor, the cooling vessel is periodically emptied of a portion of the polysilicon previously fed thereto. This emptying allows for the polysilicon in the product withdrawal tube and in the fluidized bed reactor to flow through the product withdrawal tube into the cooling vessel. After a measured amount of polysilicon has been so withdrawn, the emptying of the cooling vessel is stopped and, as a consequence, the flow through the product withdrawal tube is stopped.
All of this residence time in and movement through the product withdrawal tube would be of concern from a metals contamination standpoint if the product withdrawal tube were made of metal. The contamination specifications on high grade polysilicon allow for so little contamination that mere moving contact between polysilicon and a metal can be deleterious. This contamination problem can be obviated by constructing the product withdrawal tube from a silicon based material, say poly- or monocrystalline silicon, or by coating a substrate with polysilicon. In this way, the "contamination" is silicon contamination and is not harmful. While such provides a solution, it has been found that the solution is still short of the ideal.
It has been the practice to use a unitary polysilicon tube as the product withdrawal tube. These tubes were sealingly held in place against the gas distributor in the reactor by applying, at the bottom of the tube, a force parallel to the long axis of the tube and in a direction towards the distributor. In this way a good seal and tube support were obtained. However, these tubes were fragile owing to their silicon based construction, and fragile tubes are not favored when held in tight relationship with a gas distributor which is subject to the vibrations and shocks associated with fluidized bed processes. Unitary tubes generally have short lives due to breakage. Not only were the unitary tubes fragile, they also were expensive to construct owing to their considerable length, say 6 to 8 feet.
Therefore, there is a need for a product discharge tube which is non-contaminating to polysilicon and which has extended service life.